


my little sister's dick can't be this big

by WattStalf



Series: Commissions [373]
Category: Hunter X Hunter
Genre: Anal Fingering, Anal Sex, F/M, Role Reversal, Sibling Incest, commission
Language: English
Status: Completed
Published: 2021-02-14
Updated: 2021-02-14
Packaged: 2021-03-14 00:15:50
Rating: Explicit
Warnings: Underage
Chapters: 1
Words: 3,011
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/29410374
Author URL: https://archiveofourown.org/users/WattStalf/pseuds/WattStalf
Summary: Killua and Alluka have a routine, but she wants to try something different.
Relationships: Alluka Zoldyck/Killua Zoldyck
Series: Commissions [373]
Series URL: https://archiveofourown.org/series/1325462
Comments: 1
Kudos: 8





	my little sister's dick can't be this big

**Author's Note:**

  * For [Luka_Hellstar](https://archiveofourown.org/users/Luka_Hellstar/gifts).



Traveling with Alluka has been nice. There is such a big world out there that she knows nothing about, and even for Killua, there have been a lot of new things to see. The fact that the two of them are able to enjoy all of this together, experiencing it all hand in hand, really is a wonderful thing, and he only wishes that he had gone back for her sooner, that she could have been at his side all along.

At the very least, the two of them do all that they can to make up for lost time. She has always been clingy with him and when the two of them are alone together, in all of these unfamiliar places, it becomes that much easier for her to cling. And it becomes that much easier for their relationship to shift and change, as their efforts to make up for lost time lead to Alluka showing him just what she means when she tells him that she loves him, showing him that she sees in him the potential to make up for any and every kind of love that she may have been denied.

And for someone like Killua, who only barely hides just how lonely he is, it is easy enough to succumb to the advances of his little sister, so innocent about it all that he often wonders if she even understands what it is that she is doing with him. She must understand to some degree, at least the physical aspect, but when it comes to the implications, he is not so sure. And, at the same time, he is not so sure that he minds, just as long as he gets to keep her close, and as long as she always needs him.

Traveling without Gon at his side would have been hell without her, and he is glad that he has her, because he is not sure if he had any other option than he and Gon going their separate ways. Alluka has saved him in a lot of ways, in ways that she may never understand. Killua is always going to be grateful to his sister, always going to be happy to have her with him, and so, of course he wants to give into all of her requests and desires, doing whatever he can to keep her happy.

If that involves becoming lovers, then that is no problem for him.

But the two of them have a routine with all of that. The two of them have a set way of doing things, because Killua has a set way of carrying it out. At night, when they are alone together, he is tender with her as he helps her out of her clothes, and he lets her help him undress, even though he is pretty sure that he could get it done a lot faster if he did it all himself. Once their clothes are out of the way, he takes his time kissing her and holding her, gentle as he starts to work her up to be ready to take him.

Alluka does not have any trouble bottoming for him, especially not at this point, but Killua is always just as cautious with her as the first time, not wanting to take any chances. He can be so unintentionally clumsy with so many things, and the last thing he wants is for that to cause some sort of harm to come to his sister. She brings out a side in him so tender that he often feels like a different person when he is alone with her.

Once he has her ready, after fingering her until she is a mewling mess, he fucks her until they both get off, and then the two of them sleep together that is the routine that they follow, and Killua is set in his ways there, never thinking of any other possibility. The routine works, and they are both happy with it, and so, there never seems to be any reason to shake things up.

Until one evening, once he has her out of her clothes, his own clothes on the floor, she asks, “Can we switch tonight?”

“Huh? What do you mean?” he asks, and he really does not get it at first.

“What you always do to me,” she says, as if that is an explanation. Upon seeing his confused expression, Alluka adds, “I want to do that to you instead! Can we try something like that?”

Killua is stunned for a moment, surprised that she would ask for something like this, surprised that she would even _think_ of it. He supposes it is not actually that odd of a question to arrive at, and it only makes sense for her to be curious, but because he has been so content with how things are, and because he has never considered bottoming for even a moment, it has never registered to him that Alluka might get curious about topping.

Killua has never let anyone fuck him before, has always been on top when doing things like this. Not even Gon has ever been allowed to…but Alluka is different, and she is actually asking him, and she has that pleading look in her eyes, like this is something she really wants to do. Even if it is just to satisfy idle curiosity, she wants this, and he is not going to deny her something that she wants. He never wants to deny her anything that she wants, and so, he supposes now he just has to get used to the idea of his little sister being on top this time.

“Please?” she adds, once he has spent a bit too long thinking on it.

“Alright,” he says with a nod. “But you’re going to have to let me teach you, and you’re going to have to listen to everything I say, alright?”

“Yeah, of course!” she agrees, beaming, and he knows this was the right thing to do.

Alluka is attentive, listening closely to all of his instructions. For the most part, she has a good understanding of what to do because of the things that he has done to her in the past. She has experienced so much of it second hand that his explanations make sense, and that makes her a quick study, when it comes to getting her older brother ready to take her. She is only a little bit clumsy with it, but it still feels good, and he finds himself able to relax a bit as he lets Alluka finger him, getting him slick and ready for her.

Everything is going fine, at least at first. Killua is content how he is, and glad that he decided to let her do what she wanted. Of course, all of this is so new, and feels really weird for him, but it is not weird in a bad way. In fact, it is weird in the best way possible, and he thinks he could get used to this, if she were to decided to want to do it again. It is nice to let his little sister take care of him, a lot nicer than he would have expected, and as surprised as he is by all of this, he is still completely welcoming of it.

Alluka is a good girl, who wants to keep him happy just the same as he wants to keep her happy. She may not realize that she makes him happy just by sticking around with him, and may not realize that she does not have to try so hard to please him. There is a part of him that wonders if this might not be more than just curiosity; Alluka might think that this is the perfect way to return the favor for all of the pleasure that he has given. After all, if it feels that good for her, then surely it must feel good for her big brother as well.

Killua can’t fault her for that logic, and she isn’t even wrong, he is quick to discover. Once she has actually figured things out with her fingers, it feels really good, and he loves it, glad that she is able to pick up on it so quickly, and glad that she is able to figure out all of the things that he does for her, that make her go to absolute pieces. It will not be any trouble for him take her by now, of that he is sure, and so, he is breathless as he says, “I think that’s enough of that.”

“Are you sure?” she asks, cocking her head. “I can keep going!”

“No, no, it’s really fine,” he insists. Truth be told, he is not sure if his endurance can hold up for very long, if he just leaves Alluka to her own devices, and he would like to actually be able to let her do what he promised her that she could do. Rather than saying that, though, he just says, “Remember, you’ve got to do what I tell you, right? I know what I’m doing.”

“I know!” she agrees, and that is all that it takes to convince her. Slowly, she withdraws her fingers, and now, he wants to help her get her cock slick with the lube they have kept on them ever since this whole relationship began, not long into their journey. It is as he is getting his hand coated that it occurs to him he has never really paid much attention to her in this way. Alluka has almost always been able to get off just on having Killua bend her over and fuck her, and only every now and then has he reached around to stroke her off a bit, only when he felt like she needed it.

As he is thinking that, he turns to her, ready to start jerking her off now, and he stares for a moment, not quite believing what he is seeing. It seems odd that he would not notice before now, that they have spent so much time together without clothes, that they have been so intimate, and yet he never noticed, but as he looks at her, he knows that this is the first time he has really seen this. Alluka has grown up a lot over the years, but looking at her now, and looking between the two of them, he arrives at two unexpected conclusions.

The first is that Alluka has grown a lot below the belt, more than he noticed, and the second is that she has grown so much that she is a little bit bigger than him.

His little sister’s cock is bigger than his, and he has no idea how long _that_ has been a thing, because he was never paying attention. Definitely proof that he needs to pay closer to attention to such things, but it seems that Alluka has not noticed either, or if she has, she has not said anything. Maybe this is the sort of thing that just doesn’t register with her, and if that is the case, then there really is no reason for him to be so embarrassed about it.

Even so, as Killua starts to work his hand up and down, he can’t help but be a little embarrassed. He has no idea how to process all of this, and even though he knows it should not matter, the fact that she is younger, even by only a little bit, keeps ringing in his mind, and he blushes, hoping that she will not notice. At the same time, though, he can feel his heart racing, can feel the stabs of arousal shooting through him, and he knows that it turns him on just as much as it embarrasses him.

Yeah, he definitely has no idea how to feel about any of this, but he is in far too deep now, and it’s time to give Alluka what she wants.

He gets bent over for her just like he always has her do for him, and she gets on top of him, knowing what to do just from all of her experience in receiving. Killua does not have much more instruction to give her at this point, other than reminding her to be careful, but beyond that, this is entirely in her hands. That alone is a weird thought, the idea of Alluka taking control in any way, but he does not mind that, not nearly as much as he thought that he would, and perhaps not even at all.

When he feels the tip of her cock pressing against him, he bites his lip, wondering just how this is going to feel. He can sort of imagine it, but since it is something that he has never done before, he is not sure just how accurate his imagination is in that regard, but if he enjoys it even half as much as she seems to, then there is no way that it can be bad. That is what he tells himself, at the same time that he reminds himself that he needs to stay relaxed for this, while he lets her get into the swing of things.

Alluka is slow as she starts to push into him, taking everything that he has taught her to heart. She is slow and she is gentle with him, taking her time to fill him, and all the while, he is completely overwhelmed by every little sensation, having no idea how to process any of this. He knew that it would be weird, knew that the it would be so unfamiliar that he would have nothing else to compare it to, and yet, in the moment, he is still surprised by that, still surprised by just how weird it really feels for him.

Even more surprising than that is how much he immediately loves it. Killua is not sure how to process any of these feelings, but at the very least, he knows that those feelings are positive, that he enjoys every second of them. He can hear Alluka gasping and whimpering, can tell that she is struggling to take things slow with him, and hopes that means that it feels so good that maintaining her self-control is nearly impossible. Nearly impossible, but not quite, and she really does do her best to do everything just the way that he told her to.

Slowly and gently, she fits herself inside of him, and Killua loves it, and is so thrown by how it feels, and just how much he loves it. It feels good, and he moans beneath her, unable to stop himself from letting a bit of noise out. But there is no need to be ashamed of that, no need to hide any of his enjoyment, when he wants to encourage Alluka, wants to let her know that she is doing a good job with him.

Finally, she manages to fit herself, going still for a moment as she makes sure that he can handle it, just like he had told her to do. She whimpers on top of him, and in a breathless voice, she asks, “Is this good?”

“It’s…it’s really good,” he gasps, completely sincere. “If you want to start doing more now, you can.”

Alluka does not need to be told twice, and she starts to work her hips, gently thrusting into him, and immediately left whining as she is overcome with the pleasure that comes with this. Her whimpers are always cute, and it turns out they are just as cute when she is fucking him as they are any other time. Killua should not be surprised by that; Alluka is always going to be cute, and the fact that she may have outgrown him in one specific department definitely does not take away from that.

Actually, now that she is properly fucking him, he is a little grateful for her size, because there is no way of knowing if it would feel this good if she were any different than she is right now. Right now, Killua would have to say that his little sister is perfect, and he doesn’t care how weird all of this feels, not when he loves it this much. Despite how caught up in it all that Alluka seems to be, despite how she whimpers and whines and loses herself in her thrusts, Killua is still the first of the two of them to come.

With a sharp cry, he is suddenly overcome with his orgasm, unable to hold back for even a second, unable to even attempt to play it cool, or try and outlast his little sister. He is left panting as he relaxes beneath her, and on top of him, she continues to pound into him, desperately chasing her own high, and not taking very long to reach it. Very soon after his climax, she comes as well, filling him as she gives in, and Killua is once again overwhelmed by a very new and very weird feeling, but one that definitely feels good, one that he knows right away that he loves.

Having Alluka come inside of him is amazing, and already, he can’t stop thinking about how much he loves it, about how he would not mind letting her do this again, if only she asked. The two of them take some time to relax, letting their bliss completely overtake them as they bask in the afterglow, but she speaks up not long later, as if she read his mind before, but he is pretty sure she didn’t, pretty sure that she just loves this like he loves it, and wants it as much as he does.

“Can we do that again sometime?” she asks him, and it is not just his inability to say no to his little sister speaking anymore.

“Yes, of course,” he agrees, because he knows he wants it just as badly as she does.

**Author's Note:**

> If you're interested in my writing updates, please follow my fanfiction twitter @WattStalf


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